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Posers and Puzzles

Posers and Puzzles

  1. Subscriber sonhouse
    Fast and Curious
    14 Feb '11 17:33
    So I know if I blink my eyes as fast as I can, I am making 100 millisecond clips of time. So I blink my eyes like that at my overhead fan, and I notice I can stop the blades ala strobe effect. However, the blades are not crisp images like those strobes which flash for a few microseconds, my eyes are open for 100 millisecond clips. So in that 100 ms, the blade, which is normally 150 mm wide, is blurry, looks twice as wide. So with that info, how fast is the blade turning?
  2. 14 Feb '11 19:17
    1.5 hamster per second?
  3. Standard member forkedknight
    Defend the Universe
    14 Feb '11 21:47 / 3 edits
    I don't know about you, but I certainly can't blink my eyes 10 times/second. Therefore I think the question as posed is accurate.

    If you *could* blink your eyes 10x/second, and assuming the fan has 5 blades, it would be spinning approximately 2 RPM.

    *edit* I re-read the post, and your question may be intended differently than I read it.

    If the fan blade moves 150mm in 100ms, then the linear velocity of that section of fan blade is 1.5m/s. It is not possible to calculate the rotational velocity of the fan with the given information.
  4. 14 Feb '11 21:58
    Originally posted by forkedknight
    I don't know about you, but I certainly can't blink my eyes 10 times/second. Therefore I think the question as posed is accurate.

    If you *could* blink your eyes 10x/second, and assuming the fan has 5 blades, it would be spinning approximately 2 RPM.

    *edit* I re-read the post, and your question may be intended differently than I read it.

    If the ...[text shortened]... It is not possible to calculate the rotational velocity of the fan with the given information.
    i told you, it is the hamster!
  5. Subscriber sonhouse
    Fast and Curious
    15 Feb '11 01:19 / 1 edit
    Originally posted by forkedknight
    I don't know about you, but I certainly can't blink my eyes 10 times/second. Therefore I think the question as posed is accurate.

    If you *could* blink your eyes 10x/second, and assuming the fan has 5 blades, it would be spinning approximately 2 RPM.

    *edit* I re-read the post, and your question may be intended differently than I read it.

    If the ...[text shortened]... It is not possible to calculate the rotational velocity of the fan with the given information.
    I got that after I posted. The diameter of the fan blades is 1.3 meters. You only need one blade to make the calculation.
  6. 15 Feb '11 08:40 / 2 edits
    Originally posted by sonhouse
    So I know if I blink my eyes as fast as I can, I am making 100 millisecond clips of time. So I blink my eyes like that at my overhead fan, and I notice I can stop the blades ala strobe effect. However, the blades are not crisp images like those strobes which flash for a few microseconds, my eyes are open for 100 millisecond clips. So in that 100 ms, the bla ...[text shortened]... is blurry, looks twice as wide. So with that info, how fast is the blade turning?so it is moving
    In 100 milliseconds the blade tip moves 150 mm

    so, converting to metres and seconds:

    Velocity of tip = 0.15 / 0.1 m/s = 1.5 m/s

    blade diameter = 1.3 metres

    blade circumference = 1.3 * pi metres

    so tip will travel circumference in

    1.3 * pi / 1.5 seconds

    and the rotational speed is 1.5 /( 1.3 * pi) s^-1

    = 0.367 rotations per second, a slowly rotating fan.

    The extra constraint is that the blades must appear stopped.

    You are could be closing your eyes for 2.6 seconds, which is what would be needed for the first blade to come round again (( 1.3 * pi - 0.15)/1.5 = 2.6), but that sounds a bit long...

    So perhaps it is a 4 bladed fan and you are closing your eyes for about 0.66 seconds between snapshots?
  7. Subscriber sonhouse
    Fast and Curious
    15 Feb '11 09:52 / 1 edit
    Originally posted by iamatiger
    In 100 milliseconds the blade tip moves 150 mm

    so, converting to metres and seconds:

    Velocity of tip = 0.15 / 0.1 m/s = 1.5 m/s

    blade diameter = 1.3 metres

    blade circumference = 1.3 * pi metres

    so tip will travel circumference in

    1.3 * pi / 1.5 seconds

    and the rotational speed is 1.5 /( 1.3 * pi) s^-1

    = 0.367 rotations per second, ...[text shortened]... aps it is a 4 bladed fan and you are closing your eyes for about 0.66 seconds between snapshots?
    Yes, it is a four bladed fan. Using the tip size of 1.3 meters, I got about 45 RPM.
    I reasoned the blade as seen is blurred to twice its real size, or 300 mm, so covering twice the distance in 100 ms. In order to get the exact speed with a strobe, you would have to have two microsecond strobe pulses spaced 100 ms apart to give two blade widths because one strobe pulse will only show you the position of a single blade at a single time. You can put some marker on the blades that allows the marked blade to be picked out by the strobe pulse, stopping that blade at the same place in its revolution, then simply counting the PRF but you don't get that luxury when simply blinking your eyes at some rate, I read when eyes blink, it is on the order of 100 ms of dark time when the eye closes so I reasoned if you could do that fast enough to stop slowly rotating blades, the perceived blurring width could be used to calculate the RPM's within certain limits of course.
  8. 15 Feb '11 10:19
    Originally posted by sonhouse
    Yes, it is a four bladed fan. Using the tip size of 1.3 meters, I got about 45 RPM.
    I reasoned the blade as seen is blurred to twice its real size, or 300 mm, so covering twice the distance in 100 ms. In order to get the exact speed with a strobe, you would have to have two microsecond strobe pulses spaced 100 ms apart to give two blade widths because one ...[text shortened]... perceived blurring width could be used to calculate the RPM's within certain limits of course.
    you seem to spend a lot of time looking at that fan.

    you might enjoy this:
    YouTube

    now, giving the calculated speed and a cord length of 0.5m, and the weight of the cat of around 5kg, how much force has the wall be able to withstand?

  9. Subscriber sonhouse
    Fast and Curious
    15 Feb '11 11:58
    Originally posted by tharkesh
    you seem to spend a lot of time looking at that fan.

    you might enjoy this:
    http://www.youtube.com/watch?v=lrI7mHVHlEc

    now, giving the calculated speed and a cord length of 0.5m, and the weight of the cat of around 5kg, how much force has the wall be able to withstand?

    OMG that cat was is serious trouble. My guess is it wouldn't do that again
    I guess I am a fan of fan's
  10. Standard member smw6869
    Granny
    15 Feb '11 19:12
    Originally posted by tharkesh
    i told you, it is the hamster!
    Yes, but doesn't it depend on whether the hamster's buttocks muscle is slow twitch or fast twitch?


    GRANNY.
  11. Standard member forkedknight
    Defend the Universe
    15 Feb '11 21:43 / 1 edit
    Originally posted by sonhouse
    Yes, it is a four bladed fan. Using the tip size of 1.3 meters, I got about 45 RPM.
    I reasoned the blade as seen is blurred to twice its real size, or 300 mm, so covering twice the distance in 100 ms. In order to get the exact speed with a strobe, you would have to have two microsecond strobe pulses spaced 100 ms apart to give two blade widths because one perceived blurring width could be used to calculate the RPM's within certain limits of course.
    If the blade "appears" to be 300mm, it just means that the front edge is being replaced by the back edge, so the distance traveled is only a single width of the blade, or 150mm.

    I concur with iamatiger's estimate of .367 rps which translates to ~22 RPM
  12. 16 Feb '11 00:20 / 2 edits
    Originally posted by sonhouse
    Yes, it is a four bladed fan. Using the tip size of 1.3 meters, I got about 45 RPM.
    I reasoned the blade as seen is blurred to twice its real size, or 300 mm, so covering twice the distance in 100 ms. In order to get the exact speed with a strobe, you would have to have two microsecond strobe pulses spaced 100 ms apart to give two blade widths because one perceived blurring width could be used to calculate the RPM's within certain limits of course.
    If you did that with the strobe, it wouldn't appear stopped, the blades would appear to be advancing one blade width every flash, unless the blades were only separated by a blades width, but that would be a lot of blades.
  13. Subscriber sonhouse
    Fast and Curious
    18 Feb '11 18:15
    Originally posted by iamatiger
    If you did that with the strobe, it wouldn't appear stopped, the blades would appear to be advancing one blade width every flash, unless the blades were only separated by a blades width, but that would be a lot of blades.
    That would depend on the pulse rate. For instance, you could set up the strobe to do two quick pulses just the right time followed by a longer period, two pulses in a row with a time gap then two more pulses, you could show the same blade one blade width apart. Having said that, that is not the standard way a strobe works, it only has one pulse per period. I suggest two pulses per period would do the trick.
  14. Standard member smw6869
    Granny
    18 Feb '11 18:54
    Aren't the RPM of the fan stamped in to the hamster's left ear at the factory? Just saying.


    GRANNY.